An ultrasound imaging system transmits ultrasound signals into a target object and receives ultrasound echo signals. The echo signals are converted into electrical signals in a probe. The ultrasound imaging system performs signal processing for the electrical signals, thereby forming an ultrasound image. The ultrasound imaging system has been extensively used in the medical procession due to its non-invasive and non-destructive nature. In order to acquire a high-resolution ultrasound image, various techniques have been studied and researched in the ultrasound imaging system. Recently, an array transducer comprising a plurality of transducer elements is used to acquire the high-resolution ultrasound image. Also, the transmit focusing and receive focusing methods have been adopted for acquiring high-resolution ultrasound images.
FIG. 1 is a schematic diagram illustrating the delays of ultrasound echo signals arriving at each transducer element in an array transducer. A predetermined delay profile of transmit pulse signals is established so that ultrasound signals produced at the array transducer 10 are focused on a focal point, wherein each transducer element then produces ultrasound signals according to such predetermined delay profile. Ultrasound echo signals reflected from the focal point arrive at each transducer element in different times.
As shown in FIG. 1, an ultrasound echo signal reflected from the focal point travels a distance of “d” to arrive at a transducer element Tc, while an ultrasound echo signal reflected from the focal point travels a distance of r (r=d+Δr(x)) to reach a transducer element Tx. That is, the ultrasound echo signal received at the transducer element Tx is delayed by Δr(x) compared to the ultrasound echo signal received at the transducer element Tc. The ultrasound echo signals received at each transducer element are converted into electrical signals (hereinafter referred to as receive signals). The receive signals should be focused in order to obtain image signals. In focusing the receive signals, the delays of the ultrasound echo signals arriving at each transducer element should be compensated. A receive focusing delay technique is usually adopted to compensate for the delays of the ultrasound echo signals.
The delay of the ultrasound echo signal arriving at each transducer element can be calculated using the following equation:
                              Δτ          ⁡                      (            x            )                          =                                            Δ              ⁢                                                          ⁢                              r                ⁡                                  (                  x                  )                                                      v                    =                                                                                          x                    2                                    +                                      d                    2                                                              -              d                        v                                              (        1        )            
wherein, Δτ(x) represents a time delay of an ultrasound echo signal arriving at the transducer element Tx, ν represents a propagation speed of the ultrasound signal (speed of sound) in a target object, x represents a distance between the elements Tc and Tx and d represents a distance between the focal point and the transducer element Tc. d may be calculated using the following equation:d=νt  (2)
Wherein, t represents a time for the ultrasound signal to arrive at the transducer element Tc from the focal point.
As shown in the above equations, the speed of sound in the target object is important for determining the delays of ultrasound echo signals. When the target object is a human body, the speed of sound is generally set to 1540 m/s, which is an average speed in the soft tissues of the human body. However, the speed of sound is 1460 m/s in fat, 1555 m/s in liver, 1560 m/s in blood and 1600 m/s in muscle. That is, the speed of sound is changed according to the type of media. Therefore, if the fixed speed of sound is used, then a calculation error in the delays of ultrasound echo signals arriving at each transducer element may occur.
Recently, a compound ultrasound image is used to enhance the quality of an ultrasound image. In order to obtain the compound ultrasound image, the scan lines are steered in different angles and then ultrasound images are obtained at each angle of the scan lines. Thereafter, the obtained ultrasound images are spatially compounded, thereby forming the compound ultrasound image. In this case, if the speed of sound is not accurately set, then the delays of ultrasound echo signals arriving at each transducer element for each steered scan line cannot be accurately calculated. Thus, the pixels corresponding to an identical image of the target object may not be exactly overlapped. Therefore, the overall compound image becomes dark and a signal to noise ratio (SNR) and a contrast may be decreased.